Getting to the Fabric of Geosynthetics

Jan. 1, 2002
Mention the need to use a geosynthetic on your next project, and you might have opened the gateway to the amazing and sometimes confusing array of products that can perform a wide variety of engineering functions at your project site. The challenge is to understand which products perform what functions in order to pick the right one. Some products are designed to retain soils in place; others allow waters to flow through while reducing the amount of silt in the water flow. Some are designed to prevent any water flow, while still others help direct water through the site to minimize erosion. Understanding the needs at the site, the soil characteristics, and the desired outcome all will help in the selection process.A Geosynthetic for Every ApplicationGeosynthetics are a broad class of materials designed primarily for use in engineered earth applications. These materials are used in locations where biodegradation could be a problem and in situations in which the inherent strength and durability of the material are useful. Most geosynthetic materials used in EC applications are made of plastic, nylon, or other synthetic materials and may contain other chemical components added to create certain physical characteristics. Geosynthetic materials are divided into several different subcategories:Geomembranes. On a dollar-for-dollar basis, geomembranes are probably the largest category of geosynthetics. According to the Geosynthetic Research Institute (GRI), geomembranes are “impervious thin sheets of rubber or plastic material used primarily for linings and covers of liquid- or solid-storage facilities.” GRI notes that although “nothing is strictly impermeable,” when compared with competing materials such as natural or amended clay–substances with an impermeability of 10-7 cubic meters per second (m3/s)–geomembranes offer a much smaller diffusion permeability of 10-11 to 10-13 m3/s and are considered relatively impermeable. There are more than 30 different engineering applications for geomembranes, and these applications often are used in EC applications to line catch basins and settling ponds.Geotextiles. Geotextiles are the second largest category of geosynthetic products. Classified as textiles because of their fabriclike consistency, geotextiles consist of synthetic fibers, which are highly resistant to degradation when in contact with soil or water. Both woven and nonwoven geotextiles are manufactured. Both are porous to water flow both across and through the sheet, although the density of the weave or matting determines the porosity through the fabric. According to the GRI, at least 80 specific applications have been identified for this group of products, and determining the specific needs of the site can help determine the appropriate product.Geogrids. Unlike geotextiles, geogrids contain relatively large open spaces. Geogrids are used primarily for reinforcement, such as for soil reinforcement in the construction of retaining walls. This segment of the industry is rapidly growing, with at least 25 different applications already identified.Other geosynthetic categories include geonets or geospacers, designed to move water through a drainage area, and geosynthetic clay liners, impervious products consisting of clay sandwiched between layers of geotextile or geomembrane. These geosynthetic materials often are used at landfill sites or to prevent fluid infiltration into adjacent soils. As geosynthetic materials find new applications, geocomposites are often created, either by combining more than one geosynthetic product–a geogrid and geotextile, for example–or by combining a geosynthetic with another type of material. By combining the different products together, it is possible to create synergisms and reduce the need to use individual products to achieve the desired results. Geosynthetics, a growing area of research within the industry, produces new products and applications–designed to meet unique engineering needs–on a continuous basis.Deciding What You Need It to Do
Geosynthetics are often used to reinforce plant roots. Each subcategory of geosynthetic products is designed to perform a specific function. To select the right product, it is important to understand the product’s function or functions and the physical characteristics needed to meet those functions. Product functions can include separation, reinforcement, filtration, drainage, and creation of a moisture barrier.Separation. It is sometimes desirable to maintain a physical separation between two dissimilar materials to maximize the physical attributes of each of those materials. For example, in drainage systems, it is necessary to prevent fine soils from filling the voids in a rock base, otherwise the drainage system becomes clogged and ineffective over time. Yet it is important to allow water to pass between the soil and the drainage system. In other applications, it is desirable to prevent any water from coming into contact with the soil, so an impervious separation surface is required. The selection of an appropriate product to achieve a physical separation is determined, therefore, by the desired outcome.Reinforcement. The physical characteristics of soils, especially on slopes resulting from cuts and fill activities, create an opportunity for soil to go where you don’t want it to go. Geosynthetic products can help to strengthen the soil face and to increase the soil’s ability to stay put. As a result, slopes are stabilized either temporarily or permanently, and creep stops or at least diminishes. Also, geosynthetics can be used either to prevent water from permeating a slope or to control the amount of infiltration that occurs during various rain events. Filtration. Often it is necessary to filter out fine soil particles that are in suspension as a result of severe rain events at a site. The size of the particles, the flow rate of the water, and the physical location of the filter may determine the types of products that are appropriate. Products used as silt curtains in a flowing waterway require higher strengths to reduce failure than products used to contain occasional runoff from a construction site.Drainage. In some locations, water must be removed from a location–such as a building foundation–quickly, or flow must be directed from the face of a slope to a channel or pipe to reduce sheet erosion. In these applications, a product that has a relatively low permeability and high resistance to abrasive materials–or that has the ability to redirect water along a desired path–is necessary.Moisture Barrier. In some locations, it is important to prevent moisture from reaching certain materials, such as wood along a foundation. Although not directly applicable to erosion control, such features might be desirable in sensitive locations.Engineering a SolutionOnce it’s clear what function the geosynthetic material must perform, it is then necessary to determine the actual product or combination of products that meet the required application. It’s also necessary to consider the physical configuration of the site, soil type, and expected flow rates of water over the soil requiring protection. For most applications relating to highways and roads, the American Association of State Highway and Transportation Officials (AASHTO) has developed guidelines that have been adopted by the United States Department of Transportation of a substantial number of states. “These guidelines were set up as a joint effort between the AASHTO organization, the highway officials, and the geosynthetics industry,” states Steve Walker, a consultant with Hancor Inc. in Findlay, OH. “There are classifications, based on the difficulty of the construction site itself and the mechanical stresses to which the material would be exposed during installation.” According to Walker, a major motivation behind establishing these classifications had to do with recognizing the damage that could occur during the course of the installation. “The industry found out over the years that installation damage is a real key factor with these products,” he explains. “They have to remain intact during the installation process. For example, in an erosion control application, you have to make certain that the surface is very clear of significant stones, roots, and debris. It has to be as smooth as possible and free of depressions or holes in the soil surface. One of the key things you are trying to accomplish in erosion control applications is to make certain that the geotextile is in intimate contact with the soil itself so that it can function and do its job. Once it’s installed in place, bedding stone will be placed over the top; these materials have to be placed very carefully. It’s very easy to drop stone and rock on top of geotextiles and damage them. So we have to observe some guidelines.”Despite the focus on the AASHTO guidelines, actual requirements vary from state to state, says Jay Wilson, a technical services engineer for Linq Industrial Fabrics Inc. in Summerville, SC. “A lot of states have gone into their own testing applications, depending on the problems they may have had in the past.” Often, states have their own sets of acceptable products for certain applications on highway projects. “The engineer will specify by the state standards something on their approved products list for a [specific type of] erosion control,” explains Wilson. “The contractor knows right off what he’s got. But with private jobs, we run into a lot of specifications that just don’t make sense for the job or are from something really outdated.” Standard formulas and published information, such as the Universal Soil Loss Equation, can be used to calculate the expected erosion; this information might also form the basis for designing a solution, notes Richard Goodrum of Colbond in Enka, NC. “You can pretty much stick in numbers available to designers to come up with an answer,” he claims. “But they only are used as a general rule of thumb. For channels, we recommend a designer follow the procedures set forth in Highway Engineering Circular 15 that’s published by the Federal Highway Administration. In there are step-by-step procedures. Then you compare [that result] to what you are expecting in the channel with what is permissible.” Goodrum’s company plans to release software that helps designers calculate solutions for slopes and channels after entering the slope geometry and other parameters.Applications in the FieldOnce the desired application has been determined, the engineering has been performed, and the products have been identified, the challenge is to install the geosynthetics properly in the field. The best engineering in the world will fail if the products are installed improperly or if the materials are damaged during installation. To minimize these possibilities and to get the best results, it is desirable to use contractors who have experience installing the products and can make appropriate adjustments based on what they see in the field.Silt Fences. One common EC application for geotextiles is in silt fencing to control sediment runoff, as from a construction site. Geotextiles are ideally suited for this application because of their ability to filter suspended soils from the flow. The design and installation of these fences are a function of the expected flow rates off the site.A recent project involving silt fence installation at a newly constructed school in Washington, DC, presented a challenge for Bruce Burgess, owner of J&B Fabricators in Arnold, MD. The project demonstrates how important it is to hire people who understand the actual dynamics and proper installation of geosynthetics. “Part of the business that I like and is the most rewarding is when someone has a problem,” affirms Burgess. “They call us up and say, ‘What can we do here to make this work?’” In the school project, the original design called for standard silt fencing, which consisted of geotextile anchored to standard stakes. The project site contained steep slopes, however, so Burgess recommended a product that his company fabricates from geotextiles called Super Silt Fence. This product consists of 42-in.-high chain link fencing faced with 45-in. silt fence fabric with a 20-40 open mesh. The fabric is buried in an 8- to 10-in. trench along the foot, creating a strong, effective fence.Nature, however, stepped into the picture before the EC system could be installed. “They had a 7-inch rain in a three-and-a-half—hour period,” Burgess reports. “It rained so violently that it took some of the material that had just been excavated the day before and moved it almost 500 feet into a backyard. It did about $8,000 worth of damage to two residences from the sheet flow. We went in and recommended they put in an extra layer of Super Silt Fence. This particular job was very complex in that it ended up being over a half-mile of fence and through a lot of difficult conditions. It was a step up from the original design, which didn’t take into account how steep the slopes were and the fact that we have heavy summertime rains.”Such control measures must remain in place until the regulatory agencies are satisfied that permanent EC measures are in place and that the project has been successfully stabilized. The long-term durability of geosynthetic fabrics in such settings is particularly advantageous. “It may take a year or two,” admits Burgess. “We are removing sediment control measures from a job today that we installed over a year and a half ago. It was a very difficult site with a lot of retaining walls and steep slopes.”Burgess recognizes that the cost of installing geosynthetics at a job site can result in the use of substandard products that won’t perform. “Substandard silt fences are being distributed by people that are trying to make a buck and not paying attention to the real problems that exist,” he protests. “It’s easy to put in a very cheap fence out there. But that’s not the point. The point is that we’re trying to protect the environment and trying to do the job right. When someone buys that material, they are doing a disservice to the entire industry and to the whole cause of sediment erosion control. There’s not enough enforcement.”Riprap Installations. The installation of geotextiles under riprap is another common application, but the potential for damage is great during installation. A variety of factors must be considered, says Billy Egan of Engineered Fabrics Specialists in Norcross, GA. “Instead of just putting riprap down on bare soils where the first couple of rains will cause some of the loose settlement under the riprap to wash out, they are requiring geotextiles that keep all the fine soil particles in place and let the water run out of the riprap,” he explains.Selecting the right fabric to withstand the rigors of the installation process is critical to success. “It’s important that the fabric be designed and selected to withstand the construction damage,” maintains Egan. “That depends on the size of the riprap, the angularity, and the height from which the riprap is dropped from a backhoe or a loader bucket. All of these are considerations that the design engineer should make to ensure that the fabric is not ripped during the installation process.” The design engineer, a technical consultant, or the contractor might drive those decisions, but the vendor’s experiences will help direct the right choice.Egan expects more regulations for EC applications in the future–regulations that will fuel the development of new geosynthetic products or applications. “As they raise the education of people in the business–developers and contractors–about the things that are available to them and the different techniques they can use in the staging of projects, we’re going to see new products entering the market that try to counter the effects of stormwater pollution. Not too many years ago, there were people that did this as a sideline business. Now there are businesses for that purpose only–to provide products or services that prevent the problem from happening as opposed to trying to fix it after it’s happened.”Turbidity Curtains. Geotextiles can be installed in a flowing stream or waterway to contain any silt that actually enters the waterways. Similar to silt fences, these particular installations require additional strength because of the continual forces of the flowing water against the material. The typical turbidity curtain may consist of a flotation device installed along the top edge of the fabric, and a weight is installed along the bottom edge to keep the material in place along the streambed. “A turbidity curtain is basically a silt fence in water only,” explains Kevin Oneill, owner of GEOTK in Vancouver, WA. “You can’t pound stakes deep enough, so we put a cable under the float so that you can take some runners off that cable to keep it in place. Water really doesn’t go through it. We almost always do this in a half moon around the project site. It redirects the flow around the project site, and then the water that’s on the inside can’t get out. Water can transfer both ways, but these fabrics we make it out of are pretty fine. By the time they have worked for a while, they actually are pretty well clogged up so they can become an impermeable barrier.”Sediment Retention Bags. Another geosynthetic textile application involves the use of sediment retention bags–in essence, a large filtering bag. “Those can be anywhere from 15 to 20 feet wide and 50 to 100 feet long,” points out Oneill. The bags are often used in places where the contractor has not yet constructed a retention pond and has had a rain event. “They’ll find a place where they can collect most of the muddy water and a place to release it. We size the bag according to the time that they need to run this and the amount of flow they are going to put through the bag. If they end up with a small reservoir of extremely muddy water that they somehow have to clean up, they’ll just pump it out of that settlement area through the bag.”Once the bags have been used, their size and the layer of silt collected make removal difficult, so Oneill usually tries to locate these retention bags in places where they can be incorporated into another use. “We try to place the bag in an area that might end up with a roadway over it so that it can just stay in place,” he asserts. “The amount of silt in it, even if it’s pretty well used up, will only be 2 to 3 inches throughout the bag. They are very hard to haul off. We can either cut the top open, take that fabric away, and leave the silt in place or have it placed where it’s going to be buried and not disturbed.”The Industry Is on Solid GroundOneill notes that the industry as a whole is settling down. “Cost is becoming quite an issue, and I think the agencies are going to demand better erosion control,” he observes. “The best way to control erosion is to keep it on the slope or on the ground. Don’t allow the soil to move. There’s been a lot of energy put into trying to control it once it’s in the water or flowing over the land in water. Once you’ve reached that point, you’ve lost the battle.” He sees a movement toward an integrated approach, one that uses the various features and properties of each class of geosynthetics to maximize the total EC package at a site. “If you’re doing your job up the slope, the catch basin insert will last a long time, but when you’re trying to use it as an end-all, it doesn’t work. And all of these products are available, developed, and ready to go. It’s basically back to the basics of getting the slopes covered and planning the erosion control before the problem happens. As much as erosion control has gone forward, it’s still not being used to its potential.”